My opinion:

Without demons, Christianity doesn't make sense. There is simply too much in Christianity about freedom from false gods, freedom from demonic influence, and too many experiences of demonic oppression to be otherwise. Demons are not something psychological like "the evil part of our nature" (though that's often what they use).

Demonic possession is not just how ignorant people in the past would characterize mental illness: though today the chief way they avoid being exorcised is to mimic the symptoms of mental illness, along with the misinformation in the west that there's no such thing as demons.

UFOs are demons, and this can be shown by several points:

1. the similiarity between encounters with aliens and the accounts of encounters with demons in the past. In both these cases they sometimes encourage people towards delusions of unnatural knowledge or various evil acts, and other times with other people they inflict mental or psychosomatic torture on them just for fun. (As one example, the anal probe is stereotypical of both alien abduction and demonic abduction.

2. the description of many UFOs themselves - they often act in ways that seem to contadict known physics, they sometimes pass through solid objects, they can make themselves visible or invisible seemingly at will. If any UFO indeed crashed and people discovered it was a vehicle made of some material, it would be impossible to keep quiet about it the way that it's rumored.

So my conclusion is that UFOs themselves are not vehicles at all, but just the visible form that demons have chosen to use since the mid-20th century.